Field of the Invention
The present invention relates to a thermal type air flow meter that measures the flow rate of air, for example, to a thermal type air flow meter that is attached to an intake pipe of an internal combustion engine and measures the flow rate of intake air supplied to an engine.
Background Art
A thermal type air flow meter that measures the flow rate of intake air supplied to an internal combustion engine is attached for use to part of an intake system of an internal combustion engine. The thermal type air flow meter has a mechanism of, for example, causing a flow rate detecting element such as a heat-generating resistor to generate heat and measuring the flow rate of passing air on the basis of the amount of heat radiation therefrom to the air. Accordingly, it is necessary to consider protection of the flow rate detecting element from contaminating substances and other factors during long-term use and securement of the flow rate measuring accuracy. Further, it is also necessary to consider intake air pulsations such as a backward flow that are generated in an intake pipe of the internal combustion engine when the intake air is pulsated by opening/closing of an intake/exhaust valve of an engine and the intake pipe resonates with the rotation frequency of the engine.
In a conventional thermal type air flow meter included in an internal combustion engine, JP Patent No. 3523022 describes that a flow rate detecting element, which is placed in a secondary air passage, is protected from contaminating substances, a backward flow, and other factors. Further, in recent years, from the aspect of purification of exhaust gas and improvement of fuel efficiency, highly accurate measurement of an intake air flow rate is required, so that a thermal type air flow meter that accurately measures even a backward flow generated in an intake pipe is necessary.
In addition, with regard to a thermal type air flow meter having such a rib structure as described in the present invention, JP Patent Publication (Kokai) No. 05-302839 A (1993) proposes a structure in which a rib is provided on an element rear surface in order to reinforce resistance to vibrations of a cantilever-like element. Further, JP Patent Publication (Kokai) No. 2002-107201 A can be exemplified as a countermeasure against deformation due to a sink occurring at the time of injection molding. According to this countermeasure, such a sink occurring at the time of injection molding is reduced by providing a rib, and the degree of flatness of an opposed surface is accordingly improved, whereby air is prevented from leaking from an insertion part of a flow meter into an intake system.
A thermal type air flow meter measures a backward flow in a state where a detecting element is placed inside of a secondary air passage, and hence the secondary air passage of the thermal type air flow meter is required to have a complicated structure including a bent passage part and a narrowed-down shape. Meanwhile, a reduction in cost of the thermal type air flow meter is demanded from the market at the same time. That is, it is necessary to overcome a contradictory problem that the complicated structure is required while the reduction in cost is aimed at the same time.
In order to achieve the complicated structure of the secondary air passage, a conceivable idea involves increasing the number of components and combining the components to thereby constitute the secondary air passage. The increase in the number of components, however, leads to an increase in cost, and in order to achieve the reduction in cost, it is necessary to achieve the complicated structure without increasing the number of components. This enables not only a reduction in cost of the components but also a reduction in assembly man-hours, so that it is possible to reduce cost of the thermal type air flow meter or suppress an increase in cost thereof.
Taking the structure described in JP Patent No. 3523022 as an example, the thermal type air flow meter includes, as main components, six components of 1) the flow rate detecting element, 2) a housing, 3) a circuit board, 4) a metal base, 5) a secondary air passage member, and 6) a cover.
Among the six components, components mainly constituting the secondary air passage are two components of a portion of the metal base and the secondary air passage member. The secondary air passage member is formed by resin molding, and hence a complicated passage structure is easy to achieve.
In contrast, the metal base is a component having the portion that constitutes the secondary air passage and also having another portion that has a function of bonding and fixing the circuit board, the housing, and other components. In addition, the metal base is formed by pressing a flat plate-like metal material, and hence the metal base is advantageous to maintain a flat surface for the bonding and the like but is disadvantageous to achieve the complicated structure of the secondary air passage.
Accordingly, it is conceivable to form by resin molding only a portion of the secondary air passage constituted by the metal base member similarly to the secondary air passage member, but in order to place the flow rate detecting element inside of the secondary air passage, this portion needs to be newly prepared as a separate component, resulting in the increase in the number of components and the increase in assembly man-hours, and the increase in cost of the thermal type air flow meter cannot be avoided.
In order to solve these problems, it is conceivable to form the metal base as a resin base by resin molding, but the resin easily becomes thicker in the portion of the secondary air passage having the complicated structure, whereas the portion for bonding and fixing the circuit board, the housing, and other components is desired to be thin.
Under the circumstance, if such a thick part and such a thin part are formed as the same molded component, thermal contraction becomes uneven between the thick part and the thin part at the time of the molding, and a warpage phenomenon may occur at a boundary between the thick part and the thin part. This warpage deformation causes a trouble in bonding properties of the housing and the circuit board, and moreover, variations in a warpage amount lead to variations in a shape of a flow rate measuring unit. As a result, a new problem arises that the flow rate measuring accuracy also varies.
The present invention has been made in view of the above-mentioned points, and therefore has an object to provide a thermal type air flow meter that is capable of suppressing deformation of a base member at the time of molding, to thereby secure dimension accuracy and reduce an influence of a dimension change on measuring accuracy, thus enabling highly accurate measurement of an air flow rate.